Adhesive systems

ABSTRACT

The invention relates to the use of adhesive systems which are radically polymerizable and contain 1-30 wt.-% of a reactive solvent with a pKS value less than or equal to that of acetone and are used for the attachment of materials which are only or also cationically polymerizable to hard tissue containing water. The adhesive systems contain at least one component i) which is capable of starting a radical reaction and one component ii) which contains radically polymerizable monomers which are acid-functional or contain groups which can form acids and optionally customary additives.

This application is the national phase under 35 U.S.C. § 371 of PCTInternational Application No. PCT/EP00/07323 which has an Internationalfiling date of Jul. 28, 2000, which designated the United States ofAmerica.

The present invention relates to adhesive systems for the attachment ofmaterials which are only or also cationically polymerizable.

To date, predominantly methacrylate and acrylate monomers have been usedin polymerizable dental compositions. The2,2-bis[4-(2-hydroxy-3-methacryloyloxypropoxy)phenyl]-propane (bis-GMA)[U.S. Pat. No. 3,066,112] described by Bowen deserves particularattention. Mixtures of this methacrylate with triethylene glycoldimethacrylate also still serve today as the standard formulation fordental plastic direct filling materials. Methacryl derivatives oftwice-formylated bis-(hydroxymethyl)tricyclo[5.2.1.0^(2,6)]decane havealso proved successful as monomers for dental composites [W. Gruber etal., DE-A-27 14 538; W. Schmitt et al., DE-A-28 16 823; J. Reiners etal., EP 0 261 520]. A major disadvantage of these dental compositions ishowever the high volume shrinkage which occurs through thepolymerization. This can be minimized for example through the use ofring-opening monomers, such as the cationically curing epoxides.

Only a little is known about cationically curable epoxide compositionsfor dental applications. U.S. Pat. No. 5,556,896 describesepoxide-containing compositions which must necessarily containspiroorthocarbonates as shrinkage-compensating monomers. Bowen describesa composition containing quartz sand and an aliphatic diepoxide(bisphenol-A-diglycidyl ether) which in the cured state allegedlydisplays good stability in the mouth environment [J. Dent. Res. 35,1956, 360-379]. AT-A-204 687 describes epoxide dental compositions basedon bisphenol-A which are cured by means of Lewis acid catalysts. Thespecifications DE-A-196 48 283, WO-96/13538 and WO-95/30402 likewisedescribe polymerizable dental compositions based on epoxides and theiruse.

Although there are extensive experiences with epoxides andcycloaliphatic epoxides (U.S. Pat. No. 2,716,123, U.S. Pat. No.2,750,395, U.S. Pat. No. 2 863 881, U.S. Pat. No. 3,187,018), suchmonomers and cationically polymerizable compositions formulatedtherefrom with the properties necessary for dental applications have notbeen commercially available at any point in time.

This is because the curing of these cationically polymerizablecompositions is inhibited by water and the hard tooth substance, forexample in dentine, contains approx. 11 to 16 weight per cent water(G.-H. Schumacher et. al., Anatomie und Biochemie der Zähne, GustavFischer Verlag, 1990, 4^(th) edition).

Thus it is readily explicable that for example formulations of dentalfilling materials on an epoxides basis, in contrast to formulations on a(meth)acrylate basis, cannot show any inherent adhesion to dentine andthe use of such materials was thus not possible.

For improved attachment of (meth)acrylate-based dental fillingmaterials—i.e. radically curing systems—so-called adhesive systems areused.

The quality of these adhesive systems is reflected in the followingcriteria:

Complete bonding to the hard tooth substance without flaws (“sealing”),

Complete bonding to the filling material,

Permanent bond.

However, as the polymerization of (meth)acrylate systems takes place viaa radical mechanism, a bonding of cationically polymerizing toothmaterials to such adhesive systems is not to be expected. A dentaladhesive system for (also) cationically cross-linking materials has notbeen commercially available on the market at any point in time.

Although DE-A-197 43 564 describes compositions based on solvent-free,cationically and/or radically curable cross-linking systems asradiation-curable adhesion promoters—so-called primers—these are usedonly for the coating of anhydrous materials, for example plastics suchas polyvinylidene chloride (PVDC) or silicon.

WO-98/47046 describes photopolymerizable epoxide-based mixturescontaining an epoxide resin, an iodonium salt, a transfer moleculesensitive in visible light and an electron donor, and their use as adental adhesive system. However, it has been shown that with suchmixtures, no bonding to cationically curing mixtures is to be achievedon the hard tooth substance (see comparison mixtures 1 to 3).

WO-99/34766 states that compositions with a high proportion ofcationically curable groups do not bond at all, or only very poorly, tohard tooth tissue. To solve the problem, it is proposed to provideeither a hybrid composition containing constituents with radically andcationically polymerizable groups or a composition which is largely freefrom cationically polymerizable groups.

The object of the present invention is to provide methods of bringingeither materials which are only cationically cross-linking or materialswhich are radically and cationically cross-linking to bond to hardtissue containing water, such as a tooth, the bonding preferably beingintended to take place essentially uniformly over the entire surfaceavailable for the bonding.

This object is achieved according to the invention by the use ofadhesive systems which are radically polymerizable and contain 1 to 30wt.-%, preferably not more than 20 wt.-% and in a particularly preferredmanner not more than 15 wt.-% of a reactive solvent with a pKS valueless than or equal to that of acetone, for the attachment of materialswhich are only or also cationically polymerizable to hard tissuecontaining water, the adhesive system containing at least one componenti) which is capable of starting a radical reaction, and one componentii) which contains radically polymerizable monomers which areacid-functional or contain groups which can form acids.

Surprisingly, it was found that in the case of the use of radicallypolymerizable adhesive systems and cationically polymerizable dentalmaterials, a good bonding to hard tissue containing water, such as hardtooth substance, can be achieved, although there are two completelydifferent polymerization forms.

As the cationic polymerization takes place via an ionic chainpropagation mechanism, there are theoretically no possibilities ofpolymerizing non-radically polymerizing monomers with radically growingchains.

In addition, the water contained in the hard tissue or the reactivesolvent added to the adhesive system should disturb the cationicpolymerization, as it acts as a chain-breaking agent. Through thecontinuous chain breaking, many short chains should thus form whichprevent the construction of a polymer network.

It is completely surprising that, despite what has been said above,cationically polymerizable materials can be brought to bond to the hardtooth substance containing water or also to radically polymerizingadhesive systems containing reactive solvents. Further advantages arethe lower toxicity compared with customary solvents such as CH₂Cl₂ oracetonitrile, and the reduced volatility which allows the adhesivemixture to be applied uniformly before the solvent evaporates. Improvedsolution properties which allow the use of a large number of differentmonomers are also advantageous.

The invention is described in more detail in the following.

The adhesive mixture which displays the described advantages when usedaccording to the invention contains as constituent i) preferably 0.01 to10 wt.-%, in particular 0.1 to 7 wt.-%, and particularly preferably 0.1to 5 wt.-% of an initiator system which is capable of starting a radicalpolymerization and as constituent ii) preferably 90 to 99.99 wt.-%, inparticular 93 to 99.90 wt.-% and particularly preferably 95 to 99.90wt.-% of the radically polymerizable material.

The proportion of reactive solvent lies in the range from 1 to 30 wt.-%,preferably in the range from 5 to 25 wt.-% and quite particularlypreferably in the range from 8 to 20 wt.-%.

Reactive solvents are those with acid protons and a pKS value of lessthan or equal to that of acetone, such as water, methanol, ethanol, n-and i-propanol. In tests, it has been shown that such solvents, inparticular those which carry hydroxyl groups, contrary to what was saidabove, improve the bond between cationic materials and radicallypolymerizing materials.

The radical-forming initiators which can be contained as component i) inthe mixtures are described in the literature (e.g. J.-P. Fouassier,Photoinitiation, Photopolymerization and Photocuring, Hanser Publishers,Munich, Vienna, New York, 1995 or else J.-P. Fouassier, J. F. Rabek(Publ.), Radiation Curing in Polymer Science and Technology, Vol. II,Elsevier Applied Science, London, New York, 1993). They can also besubstances activatable by UV or visible light, such as benzoin alkylethers, benzil ketals, acylphosphinic oxides or aliphatic and aromatic1,2-diketone compounds, for example camphorquinone, the catalystactivity being able to be accelerated through the addition ofactivators, such as tertiary amines, for example dialkylamino-4-benzoicacid ester, or organic phosphites, in per se known manner.

Suitable initiator systems for the triggering of the radicalpolymerization via a redox mechanism are for example the peroxide/amine,peroxide/barbituric acid derivatives or peroxide/acids systems. Whenusing such initiator systems it is expedient to keep an initiator (e.g.peroxide) and a catalyst component (e.g. amine) separate. The twocomponents are then mixed together homogeneously shortly before theiruse.

The customary monomers used in radically curing dental materials can beused as component ii), component ii) having to contain 3 to 100 wt.-%radically polymerizable monomers which are acid-functional, or containgroups which can form acids with water, such as for example acidchlorides or anhydrides, there being meant by acids mono-, di- orpolycarboxylic acids with the following radicals: C₁ to C₂₅ alkyl orcycloalkyl radicals optionally substituted with N, O, S, Si, P orhalogen, aromatic C₆ to C₁₂ radicals or heterocyclic C₃ to C₁₂ radicalssubstituted with N, O, S, P and optionally with halogen. In just thesame way, acids such as 4-methacryloxy-ethyl trimellitic acid or itsanhydrides (Takeyama, M. et al., J.Jap.Soc. f. Dent. App. A. Mat. 19,179 (1978)) or the reaction products of trimellitic acid chlorideanhydride can be used with aminic, thiolic or hydroxylic (meth)acrylicacid esters, such as for example 2-hydroxyethylene methacrylate (HEMA)or methacroyloxy-ethyl-o-phthalate.

Other preferred acids are unsaturated organic esters ofmonofluorophosphonic acids, as described in U.S. Pat. No. 3,997,504,unsaturated organic esters of acids of phosphorus which contain chlorineor bromine directly bound to the phosphorus, as described in EP-A-0 058483, unsaturated organic esters of phosphoric acid which are present ascyclic pyrophosphates (anhydrides) as described in DE-A-3 048 410 andunsaturated organic esters of phosphoric or phosphonic acids asdescribed in DE-A-2 711234 and DE-A-3 150 285. Just as preferred are thehydrolysis-stable, polymerizable acrylic phosphonic acids of DE-A-1 974670 8.

Quite particularly preferred are ethylenically unsaturated carboxylicacids of the formula:

in which:

R¹, R², R³=H, C₁ to C₂₅ alkyl or cycloalkyl radicals, optionallysubstituted with N, O, S, Si, P or halogen, or aromatic C₆ to C₁₂radicals or heterocyclic C₃ to C₁₂ radicals containing N, O, S, P andoptionally substituted with halogen,

as well as ethylenically unsaturated phosphoric acid esters of thefollowing formula:

in which:

X=O, S;

R⁴ and R⁵ independently of each other stand for H, OH or C₁ to C₂₅ alkylor cycloalkyl, optionally substituted or bridged with heteroatoms suchas N, halogen, Si, O or S, aromatic C₆ to C₁₂ and/or heterocyclic C₄ toC₁₂ radicals, or substituted with acrylic acid esters, the radicals R⁴and R⁵ also independently of each other being able to be bound to thephosphorus via O, or

where

R⁶ stands for hydrogen or C₁ to C₆ alkyl,

n is an integer ≧1 and

A is a divalent C₁ to C₂₅ alkylene or cycloalkylene radical, optionallysubstituted or bridged with N, O, S, Si, P or halogen, or an aromatic C₆to C₁₂ radical, and/or heterocyclic C₄ to C₁₂ radical with N, O, S, or Pand optionally substituted with halogen, provided that the group whichcontains the radical R⁶ is present at least once.

Typical monomers or prepolymers which cure according to the radicalchain mechanism and can also be present in component ii) are acrylatesor methacrylates. Suitable are mono- and polyfunctional (meth)acrylatemonomers. Typical representatives of this compound class (see DE-A-4 328960) are alkyl(meth)acrylates, including the cycloalkyl(meth)acrylates,aralkyl(meth)acrylates and 2-hydroxyalkyl(meth)acrylates, for examplehydroxypropyl methacrylate, hydroxyethyl methacrylate, isobornylacrylate, isobornyl ethacrylate, butylglycol methacrylate, acetylglycolmethacrylate, triethylene glycol methacrylate, polyethylene glycoldimethacrylate, 2-phenylethyl ethacrylate, 2-ethylhexyl ethacrylate,cyclohexyl methacrylate, lauryl methacrylate and hexanedioldi(meth)acrylate.

Long-chained monomers as described in U.S. Pat. No. 3,066,112 based onbisphenol-A and glycidyl methacrylate or their derivatives produced bythe addition of isocyanates can also be used. Also suitable arecompounds of the type bisphenyl-A-diethyloxy (meth)acrylate andbisphenol-A-dipropyloxy (meth)acrylate. The oligoethoxylated andoligopropoxylated bisphenol-A-diacrylic and dimethacrylic acid esterscan also be used. Also well suited are the diacrylic and dimethacrylicacid esters of bis(hydroxymethyl)-tricyclo[5.2.1.0^(2,6)]-decane namedin DE-A-2 816 823 and the diacrylic and dimethacrylic acid esters of thecompounds of the bis(hydroxymethyl)-tricyclo[5.2.1.0^(2,6)]-decaneextended with 1 to 3 ethylene oxide and/or propylene oxide units.Mixtures of the named monomers can also be used.

The adhesive systems according to the invention can also containfillers, dyes, flow modifiers, stabilizers, solvents, ion-emittingsubstances, bactericidal or antibiotic substances which containX-ray-opacity-increasing compounds or further modifiers.

Suitable as fillers are for example substances such as are used incustomary dental materials, particularly preferably quartz, aerosils,highly-dispersed silicic acids, organic fillers or glasses, such as areused in composites customary on the market, or mixtures of thesesubstances or also those such as are described in DE-A-196 48 283 A1(page 10, lines 48-59).

Of the ion-emitting substances, those are preferred which make possiblethe release of fluoride ions, such as fluoride salts of the first orsecond main groups, such as sodium fluoride or calcium fluoride, orcomplex fluoride salts, such as. KZnF₃, or such as are described inEP-A-0 717 977, fluoride-emitting glasses, as well as mixtures of thesefluoride ion sources.

There can be used as bactericidal or antibiotic substances, for example,chlorohexidine, pyridinum salts or the customary pharmaceuticalsubstances, such as β-lactam antibiotics (penicillins), cephalosporins,tetracyclins, ansamycins, kanamycins, chloroamphenicol, fosfomycin,antibacterial macrolides, polypeptide antibiotics, chemotherapeutics,such as sulphonamides, dihydrofolate reductase inhibitors, nitrofuranderivatives or gyrasis inhibitors.

The adhesive mixtures according to the invention can also containdiluents, preferably solvents such as dialkyl ketones (e.g. acetone,methyl ethyl ketone), acetylacetone or alcohols (e.g. ethanol, propanol)or also thin-flowing polymerizable substances such as 2-hydroxyethylmethacrylate.

There can be used as stabilizers, for example, radical captors such ashydroxybenzenes or HALS (hindered amines light stabilizers).

In addition, at least one non-polymerizable acid can also be contained,such as an unsaturated carboxylic acid, phosphoric acid, phosphonicacid, sulphuric acid, sulphinic acid, sulphenic acid, mineral acid,Lewis acid or complex acid, such as for example H₂PtCl₆.

If the adhesive mixtures contain additives in addition to the componentsi) and ii), these can be present individually or mixed in the quantitiesof 0.1 wt.-% to 85 wt.-%, the mixture being prepared such that with thecomponents i) and ii) they add up to 100 wt.-% in total.

The adhesive mixture comprises for example the following constituents:

i) a radical initiator system in a quantity of 1 to 5 wt.-%;

ii) radically polymerizable monomers which are acid-functional orcontain groups which can form acids, in a quantity of 3 to 99 wt.-%;

iii) radically polymerizable monomers which are not acid, in a quantityof 0 to 90 wt.-%;

iv) solvents in a quantity of 1 to 75 wt.-%;

v) a reactive solvent in a quantity of 1 to 30 wt.-%;

vi) fillers in a quantity of 0 to 75 wt.-%;

vii) bactericidal substances or preservatives in a quantity of 0 to 20wt.-%;

viii) ion-emitting substances in a quantity of 0 to 25 wt.-%;

ix) stabilizers in a quantity of 0 to 10 wt.-%;

x) non-polymerizable acids in a quantity of 0 to 25 wt.-%.

The instant invention also comprises a kit comprising a) an adhesiveslustem, containing a component i), which is capable of starting aradical reaction and a component ii), which contains radicallypolymerizable monomers, which are acid-functional or contain groups,which can form acids, iii) 1 to 30 wt.-% of a receive solvent with a pKSvalue less than or equal to that of acetone an b) a material which isonly or also cationically polymerizable. Moreover, the kit optionallyhas component iii) stored separately from the other components of thekit.

In the following, the invention is explained in more detail by means ofexamples, these to be understood as being embodiments and not limitingin any way.

Bonding Measurement on Bovine Teeth Through Adhesive Attachment of aFilling Material:

The adhesion bond was tested by a peel-off test on bovine teeth. Pertest, 5 freshly extracted bovine teeth were ground down by means of sandpaper until there was a sufficiently large exposed dentine surface. Waxplatelets with a punched-out hole measuring 6 mm were glued onto each ofthese surfaces to obtain a standardized bonding surface. The furthertreatment of the test surface and the application of the adhesivemixture were in each case as indicated in the preparation examples.

Preparation of a Cationically Curing Filling Material:

In a three-finger kneader, the following constituents are kneaded into ahomogeneous paste. There are used for 100 g paste:

75.000 wt.-% (75.000 g) quartz (average grain size 0.9 μm, was silanizedwith 5 wt.-% glycidyloxypropyltrimethoxysilane);

0.525 wt.-% (0.525 g)4-methylphenyl-4-isopropylphenyl-iodoniumtetrakis-(penta-fluorophenyl)borate;

0.223 wt.-% (0.223 g) camphorquinone (Merck, Darmstadt);

0.001 wt.-% (0.001 g) ethyl-4-dimethylaminobenzoate (Merck, Darmstadt);

0.001 wt.-% (0.001 g) 2-butoxyethyl-4-dimethylaminobenzoate;

12.125 wt.-% (12.125 g) 3,4-epoxycyclohexyl-3,4-epoxycyclohexanecarboxylate;

12.125 wt.-% (12.125 g)1,3,5,7-tetrakis-(2,1-ethanediyl-3,4-epoxycyclohexyl)-1,3,5,7-tetramethylcyclotetrasiloxane.

Preparation of Adhesive Mixture 1 According to the Invention:

For the preparation of 10 g of adhesive mixture 1, the followingconstituents are mixed together intensively:

22.300 wt.-% (2.930 g) 2-hydroxyethyl)-methacrylate (Merck, Darmstadt);

7.000 wt.-% (0.700 g) ethanol;

1.200 wt.-% (0.120 g) camphorquinone (Merck, Darmstadt);

0.900 wt.-% (0.090 g) ethyl-4-dimethylaminobenzoate (Merck, Darmstadt);

68.600 wt.-% (6.860 g) 4-methacryloxyethyl trimellitic acid (see U.S.Pat. No. 4,148,988).

Preparation of Adhesive Mixture 2 According to the Invention:

For the preparation of 10 g of adhesive mixture 2, the followingconstituents are mixed together intensively:

19.600 wt.-% (1.960 g) 2-hydroxyethyl methacrylate (Merck, Darmstadt);

1.200 wt.-% (0.120 g) camphorquinone (Merck, Darmstadt);

0.900 wt.-% (0.090 g) ethyl-4-dimethylaminobenzoate (Merck, Darmstadt);

68.300 wt.-% (6.830 g) methacryloyl-oxydecyl-phosphate;

10.000 wt.-% (1.000 g) water.

Preparation of Adhesive Mixture 3 According to the Invention:

For the preparation of 10 g of adhesive mixture 1, the followingconstituents are mixed together intensively:

5.500 wt.-% (0.550 g) (2-hydroxyethyl)-methacrylate (Merck, Darmstadt);

1.200 wt.-% (0.120 g) camphorquinone (Merck, Darmstadt);

0.900 wt.-% (0.090 g) ethyl-4-dimethylaminobenzoate (Merck, Darmstadt);

25.000 wt.-% (2.500 g) 4-methacryloxyethyl trimellitic acid (see U.S.Pat. No. 4,148,988).

55.000 wt.-% (5.500 g) methacryloyl-oxydecyl-phosphate;

12.400 wt.-% (1.240 g) water.

Preparation of Comparison Mixture 1:

For the preparation of 10 g of comparison mixture 1, the followingconstituents are mixed together intensively:

97.300 wt.-% (9.730 g)1,3,5,7-tetrakis-(2,1-ethanediyl-3,4-epoxycyclohexyl)-1,3,5,7-tetramethylcyclotetrasiloxane;

2.0 wt.-% (0.200.9) Rhodorsil PI 2074 (Rhone Poulenc, iodonium salt);

0.500 wt.-% (0.50 g) camphorquinone (Merck, Darmstadt);

0.200 wt.-% (0.020 g) BEDB (Lambson).

Preparation of Comparison Mixture 2:

For the preparation of 10 g of comparison mixture 2, the followingconstituents are mixed together intensively:

97.300 wt.-% (9.730 g) 3,4-epoxycyclohexyl-3,4-epoxycyclohexanecarboxylate;

2.0 wt.-% (0.200 9) Rhodorsil PI 2074 (Rhone Poulenc, iodonium salt);

0.500 wt.-% (0.050 g) camphorquinone (Merck, Darmstadt);

0.200 wt.-% (0.020 g) BEDB (Lambson);

Preparation of Comparison Mixture 3:

For the preparation of 10 g of comparison mixture 3, the followingconstituents are mixed together intensively:

48.650 wt.-% (4.865 g) 3,4-epoxycyclohexylmethyl-3,4-epoxycyclohexanecarboxylate;

48.650 wt.-% (4.865 g)1,3,5,7-tetrakis-(2,3-ethanediyl-3,4-epoxycyclohexyl)-1,3,5,7-tetramethylcyclotetrasiloxane:

2.000 wt.-% (0.200 g) Rhodorsil PI 2074 (Rhone Poulenc, iodonium salt);

0.500 wt.-% (0.050 9) camphorquinone (Merck, Darmstadt);

0.200 wt.-% (0.020 9) BEDB (Lambson).

Method:

The test surface was etched for 20 seconds following the procedurecustomary in practice, using a customary phosphoric acid solution(Atzgel Minitip®, ESPE Dental AG, Seefeld), and then rinsed with water.A quantity of the test mixtures sufficient for complete wetting of thetest surface was worked into the thus-prepared dentine surfaces with amicrobrush for 20 seconds, blown briefly with compressed air andpolymerized for 20 seconds by means of a light-polymerization device(Elipar Highlight®, ESPE). The cationically curing filling material, thepreparation of which is described above, was then introduced into theholes of the wax platelets and fully polymerized by 40 seconds' exposureto light. The testpieces were then removed in a tensile test (ZwickUniversal Test Machine).

The bonding values measured can be seen in Table 1.

TABLE 1 Bonding of the adhesive mixtures described in the examples:Adhesive mixture Dentine bonding [Mpa]* Adhesive mixture 1 3.6 Adhesivemixture 2 4.2 Adhesive mixture 3 3.8 Pertac Universal Bond (ESPE) 2.2Prime & Bond NT (Dentsply) 1.9 Scotchbond Multi Purpose Plus (3 M) 2.1Visio Bond (ESPE) 0.0 Comparison mixture 1 0.0 Comparison mixture 2 0.0Comparison mixture 3 0.0 *Average from 5 measurements each

The above table shows that with the cationically polymerizing comparisonmixtures 1 to 3, no bonding to the hard tooth substance containing watercan be achieved. On the other hand, it is shown that with the radicallypolymerizing adhesive mixtures 1 to 3 according to the invention andalso with the shown radically polymerizing commercially availableadhesive systems according to the invention, cationically polymerizabledental materials can be adhesively attached to a given substance.

What is claimed is:
 1. A method of attaching materials which arecationically polymerizable on hard tissue containing water, the methodcomprising using an adhesive system to join the material to the hardtissue; the adhesive system is radically polymerizable and contains 1 to30 wt.-% of at least one reactive solvent with a pKS value less than orequal to that of acetone, the adhesive system containing at least onecomponent i) which is capable of starting a radical reaction, and atleast one component ii) which contains 3 to 100 wt.-% radicallypolymerizable monomers which are acid-functional or contain groups whichcan form acids.
 2. The method according to claim 1, wherein the reactivesolvent carries hydroxyl groups.
 3. The method according to claim 1,component i) being present in proportions from 0.01 to 10 wt.-% andcomponent ii) in proportions of 90.00 to 99.99 wt.-%.
 4. The methodaccording to claim 1, the materials being dental materials and the hardtissue a tooth.
 5. The method according to claim 1, the adhesive systemcontaining as component ii) a singly or repeatedly ethylenicallyunsaturated organic acid or its anhydride or its acid chloride.
 6. Themethod according to claim 1, wherein the adhesive system also containsone or more of the following components selected from the groupconsisting of: a further radical and/or cationic polymerizationinitiator; a diluent, a filler such as is used in customary dentalmaterials; a fluoride ion source; an acid which contains no double bond;and a bactericidal agent or customary antibiotic.
 7. The methodaccording to claim 6, wherein the diluent is an organic solventcustomary in adhesive mixtures or a thin-flowing compound which containsradically polymerizable groups.
 8. The method according to claim 1, theadhesive system being polymerized through the supply of electromagneticradiation.
 9. The method according to claim 8, wherein electromagneticradiation is irradiation with light of a wavelength from 350 to 1000 nm.10. The method according to claim 1, component ii) containing anethylenically unsaturated carboxylic acid of the following formula:

in which: R¹, R², R³=H, C₁ to C₂₅ alkyl or cycloalkyl radicals,optionally substituted with N, O, S, Si, P or halogen, or aromatic C₆ toC₁₂ radicals or heterocyclic C₃ to C₁₂ radicals with N, O, S, P andoptionally substituted with halogen, or contains an ethylenicallyunsaturated phosphoric acid ester of the following formula:

in which: X=O, S; R⁴ and R⁵ independently of each other stand for H, OHor C₁ to C₂₅ alkyl or cycloalkyl, optionally substituted or bridged withheteroatoms, such as N, halogen, Si, O or S, aromatic C₆ to C₁₂ and/orheterocyclic C₄ to C₁₂ radicals, or substituted with acrylic acidesters, the radicals R⁴ and R⁵ also independently of each other beingable to be bound to the phosphorus via O, or

where R⁶ stands for hydrogen or C₁ to C₆ alkyl, n is an integer ≧1 and Ais a divalent C₁ to C₂₅ alkylene or cycloalkylene radical, optionallysubstituted or bridged with N, O, S, Si, P or halogen, or an aromatic C₆to C₁₂ radical, and/or heterocyclic C₄ to C₁₂ radical with N, O, S, or Pand optionally substituted with halogen, provided that the group whichcontains the radical R⁶ is present at least once.
 11. The methodaccording to claim 1, whereby the hard tissue is a hard tooth substanceand after applying the adhesive system and before joining the materialto the hard tooth substance, coating is carried out with anothermaterial which is only or also cationically polymerizable.
 12. Kitcomprising a) an adhesive system, containing a component i), which iscapable of starting a radical reaction and a component ii), whichcontains radically polymerizable monomers, which are acid-functional orcontain groups, which can form acids, iii) 1 to 30 wt.-% of a reactivesolvent with a pKS value less than or equal to that of acetone and b) amaterial which is only or also cationically polymerizable.
 13. Kitaccording to claim 12, component iii) being stored separately from theother components.